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arXiv 1303.0852math.GT
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四维流形上的不动点自由圆作用

Fixed-point free circle actions on 4-manifolds

Weimin Chen

摘要:

本文研究可定向四维流形上的不动点自由$S^1$作用(光滑或局部线性).我们证明了基本群在这类4-流形的等变分类中起着主导作用。特别地,证明了对于任意无穷中心的群,至多存在1/2个不同的光滑(分别为.拓扑的)4-流形,支持一个不动点自由光滑(分别)。局部线性)$S^1$ -作用并将给定群实现为基本群。在基本群上的不动点自由$S^1$ -作用在某些进一步的条件下的等价类的数目也是如此。$S^1$ -流形的分类和基本群之间的联系是由$S^1$ -流形的某种分解给出的,称为纤维和分解。更具体地说,每个纤维和分解自然地引起基本群的Z分裂。在本文中有两个技术结果在我们的考虑中起着核心作用。一种是Z-分裂是基本群在Rips和Sela意义下的正则JSJ分解。另一种观点认为,如果基本群有无穷远中心,则4-流形上任何不动点自由$S^1$作用的主轨道的同伦类必定是无穷远的,除非4-流形是某个椭圆3-流形的周期反同态的映射环面.本文最后讨论了关于光滑分类的拓扑性质和具有光滑不动点自由$S^1$作用的4-流形的Seiberg-Witten不变量的两个问题。

英文摘要:

This paper is concerned with fixed-point free $S^1$-actions (smooth or locally linear) on orientable 4-manifolds. We show that the fundamental group plays a predominant role in the equivariant classification of such 4-manifolds. In particular, it is shown that for any finitely presented group with infinite center, there are at most finitely many distinct smooth (resp. topological) 4-manifolds which support a fixed-point free smooth (resp. locally linear) $S^1$-action and realize the given group as the fundamental group. A similar statement holds for the number of equivalence classes of fixed-point free $S^1$-actions under some further conditions on the fundamental group. The connection between the classification of the $S^1$-manifolds and the fundamental group is given by a certain decomposition, called fiber-sum decomposition, of the $S^1$-manifolds. More concretely, each fiber-sum decomposition naturally gives rise to a Z-splitting of the fundamental group. There are two technical results in this paper which play a central role in our considerations. One states that the Z-splitting is a canonical JSJ decomposition of the fundamental group in the sense of Rips and Sela. Another asserts that if the fundamental group has infinite center, then the homotopy class of principal orbits of any fixed-point free $S^1$-action on the 4-manifold must be infinite, unless the 4-manifold is the mapping torus of a periodic diffeomorphism of some elliptic 3-manifold. The paper ends with two questions concerning the topological nature of the smooth classification and the Seiberg-Witten invariants of 4-manifolds admitting a smooth fixed-point free $S^1$-action.

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